1. Field of the Invention
The field of art to which this invention relates is electronic packaging. More specifically, this invention relates to a method and apparatus for insulating moisture sensitive plastic ball grid array (PBGA) modules during attachment or removal from circuitized substrates.
2. Description of the Related Art
Typically, one or more semiconductor chips, or other such electronic devices, are mounted on a first circuitized substrate (called a semiconductor chip carrier, or more generally, a first level electronic package or module), which, in turn, is mounted on a second circuitized substrate such as a printed circuit card or board (PCB) (more generally called a second level electronic package or module). The electronic devices mounted on the first level electronic module are electrically connected through the circuitry of the first level module to the circuitry of the second electronic module. The resulting structure may be used as part of a computer or other such equipment.
A semiconductor chip, or other such electronic device, may be mounted on pads, which are part of the circuitry formed on the chip carrier. Conventional techniques are used to mount the chip carrier to the semiconductor chip. One such technique is called solder reflow which uses solder bumps, such as solder balls corresponding to the pads. With each contact pad on the chip carrier being positioned on the appropriate solder bump on the chip, the assembly is heated so as to liquefy the solder and bond each contact pad on the chip carrier to the confronting solder bump on the chip. The pads and bumps, or balls, are typically positioned in an area array called a ball grid array (BGA). The chip, chip carrier and BGA together form the first level electronic package, or module. The first level module is then attached to a PCB in much the same way, using a grid of solder balls corresponding to an array of conductive pads on the PCB.
Plastic ball grid array (PBGA) modules are moisture sensitive. The specification for attaching or removing PBGA's from PCB's requires that the solder balls on the bottom side of the PBGA reach approximately 200.degree. C. while a cover plate on top of the chip cannot exceed 220.degree. C. during reflow to attach a placed good PBGA or to remove a defective PBGA from the PCB. Exposure to a temperature exceeding 220.degree. C. can result in damage to the chip.
Difficulty arises because of the design limitations of the rework tool used in such procedures. The printed circuit board (PCB) is placed in a bias bay that heats the PCB and its first level modules somewhere between room temperature and 135.degree. C. The bias bay is limited to heating a PCB up to 150.degree. C. uniformly. In order to heat the solder joints (in the case of removal) or solder balls (in the case of attachment) between the PBGA and the PCB, hot gas must be applied to the top side of the PBGA. The hot gas temperature required is a minimum of 230.degree. C.
The problem arises because a cover plate affixed on the PBGA is the first part of the structure that sees the hot gas. While the solder joints, or balls to be melted are many layers below in the structure. The tool design makes it impossible to heat the solder joints, or balls to 200.degree. C. while maintaining the cover plate temperature below 220.degree. C. Double sided product (PBGA's on both the top and bottom of the card) increase the difficulty in reworking the site. Additionally, because of the hot gas flow across the chip, the solder balls joining the chip to the carrier on the first level module can reflow during attachment or removal of the first level module to the PCB. This can cause opens between the chip and carrier or excessive intermetallic growth that will eventually lead to solder joint failure.